1. Field of the Invention
The present invention relates generally to diagnostic imaging and more particularly relates to methods and apparatus for optical imaging of tissue containing a luminous object, such as a section of diseased tissue with an applied contrast agent.
2. Description of the Related Art
In many current modalities for medical imaging (i.e. X-ray, PGT, and CAT tomography), contrast agents have been used to enhance image quality and the resulting volume of information. Contrast agents have been introduced in optical imaging to enhance the ability to take an image of an object hidden in a scattering medium or tissue and improve the resulting image quality. An object located inside a scattering medium, such as tissue, is made luminous and viewed using its emission light by introducing a fluorescence dye or inorganic phosphorescence agent. Improvement in the image quality is achieved due to the fact that the distance that the emitted light by the luminous object traverses from the object to the exit point of the scattering medium is shorter than the distance that the illuminating light has to traverse in transillumination techniques. Further improvement of the image quality has been demonstrated by selectively reducing the diffusive component of the emitted light by the luminous object reaching the detector by introducing an absorbing dye into the random medium or using polarization difference imaging.
When tissue is illuminated with light, a first portion of the light undergoes elastic scattering, a second portion of the light undergoes inelastic scattering and a third portion of the light is absorbed by the tissue. When a tissue molecule absorbs a photon, it acquires an amount of energy that it will release within a time interval of less than 1 second. The excess energy of the photo excited tissue molecules is released via nonradiative processes giving rise to generation of phonons or via radiative processes giving rise to light emission by the tissue. FIG. 1A shows native light emission by breast chicken tissue illuminated with a 632 nanometer (nm) source. The emissions from the tissue extend up to 950 nm covering a significant part of the "spectral window" that may be used for optical biomedical imaging (700-1200 nm). The temporal profile of the emission shown in FIG. 1A is shown in FIG. 1B. The temporal profile extends to over 2 nanoseconds showing that the recorded light is due to emission by photo excited tissue molecules without any contrast agents.
Contrast agents for use in medical imaging are specifically selected to bind to molecules associated and/or involved in tumors, cancers, brain disorders, liver disorders or other disorders or diseases of the human body. The contrast agents may be injected into the human body to reach and concentrate mostly in the diseased part of the body. Optical images of the diseased part of the human body may then be attained provided that the diseased part of the body can be illuminated and the emitted light from the contrast agent can be recorded by a detection/imaging system.
When the concentration of the contrast agent located at the diseased part of the human body is very small (giving rise to very weak emission) or when the object is deep inside the tissue and it is difficult to illuminate and/or record the emitted light (due to the thickness of the tissue between the diseased tissue and the exterior of the tissue), the detection of the emitted light by the luminous object may be orders of magnitude less than the emitted native fluorescence light by the normal tissue. This effect makes the detection of the emission from the diseased part of the body very difficult or impossible.
Therefore, there remains a need for apparatus and methods that address these problems and enhance the visibility of a luminous object inside a sample of tissue.